Experiments will be performed on isolated cardiac tissues from rabbit and dog. The objective is to obtain comparative basic information on the electrophysiology of normal and diseased cardiac pacemaker fibers as well as to determine the role of spontaneous accumulation and depletion of potassium and calcium in the extracellular space in modulating automaticity and conduction. Potassium and calcium ion sensitive microelectrodes will be used to record ionic activities in the extracellular space of sinoatrial node, atrioventricular node and His-Purkinje system as well as Purkinje fibers from infarcted myocardium. Conduction and automaticity will be evaluated simultaneously using standard microelectrodes to record transmembrane potential. The extracellular ionic transients and transmembrane potentials will be compared during spontaneous rhythm, overdrive suppression, and autonomic stimulation. The experiments are designed to provide electrophysiologic data which have a direct bearing on the mechanisms of cardiac arrhythmias. In addition, the experiments will provide baseline data on animal models which can be used to evaluate the efficacy of various therapeutic interventions.